…648)

* feat: Add support for SEP-2575

* chore: remove error logs

* chore: fix presubmits

* chore: add todo

* feat(core): support DRAFT-2026-v1 stateless transport and version negotiation retry limit

* style: format python transport files with black and isort

* refactor: move Protocol import to the top of the file

* test: filter out draft versions in Go E2E tests, add todo remarks, and update protocol unit test expected versions

* docs: add error requirements docstring in conformance client script

* fix(core): update stateless protocol version retry logic to use spec-compliant -32004 code

* fix(core): align ClientCapabilities Pydantic schema with spec

* refactor: simplify version negotiation retry logic in MCP transport

* chore: delint

* refactor(core): implement transport proxy for protocol fallback

* chore: delint

* fix: fix integration tests

* chore: delint

* chore: fix license header

Co-authored-by: Averi Kitsch <akitsch@google.com>

* chore: remove unused retry logic from inside the transport class

* chore: delint

---------

Co-authored-by: Averi Kitsch <akitsch@google.com>

* test: upgrade integration harnesses for dual-server execution

* chore: delint

* test: update E2E protocol fallback assertions to support dynamic server URLs and ignore noise warnings in tool tests

* chore: update package dependencies and metadata across all sub-packages

* refactor: remove platform-specific .exe extension from toolbox binary path resolution in test configurations

* refactor: convert patch_toolbox_client_url to an explicit fixture and apply it to E2E tests via pytestmark

* refactor: reorder imports and constants in test conftest files for consistency

* fix: remove autouse from patch_toolbox_client_url fixture to prevent unnecessary patching in integration tests

* feat(core): add support for MCP 2026 stateless draft and auto-negotiation fallback (#704)

* feat(core): add custom protocols array support for ToolboxClient

* resolve PR 707 review comments

* remove redundant constant definitions in conftest.py

* test: fix conftest import locations and update readme links

* refactor: consolidate fallback mechanism tests into test_client.py and remove redundant test_fallback.py

* chore: delint

* refactor: centralize test constants into package-specific files and update imports across test suites

* test: update integration tests to use stable server URL constant

* refactor: remove redundant pytestmark fixture declaration in integration tests

* test: update integration tests to use TOOLBOX_SERVER_URL_STABLE for server connections

* chore: delint

* fix: add comments for dual-server to test fixtures

* fix: update protocol negotiation error code from -32004 to -32022 across all transport versions

* chore: delint

* feat: implement automated MCP protocol version negotiation for old MCP versions

* fix: update error code to -32022 for detecting server-supported protocol versions

* fix: parse standard json-rpc errors on http 400

* feat: add automated protocol version fallback negotiation for invalid protocol version errors across all MCP transports

* refactor: fix integration tests by replacing aioresponses with AsyncMock

* test: update exception match regex to include MCP invalid request code in e2e tests and add debug script

* test: use try/except for flexible auth error matching in e2e tests

* fix(core): handle json-rpc error dict for protocol version mismatch

* fix(core): match unsupported protocol version in error dict and fix lint

* Fix unit tests related to _meta JSON error parsing, mock misconfiguration, and version negotiation array index checks

* style: Run black and isort to fix lint CI errors

* style: Run black across all packages

* style: fix isort formatting with black profile

* style: reformat imports per package to respect first-party modules

* refactor: extract -32022 to constant and add missing fallback tests

* style: move UNSUPPORTED_PROTOCOL_VERSION_ERROR_CODE to top of types.py

* Extract -32022 into constant for v20260618 transport

* Add missing test_version_negotiation_raises_fallback to legacy versions